1. Field of the Invention
The present invention relates to the field of switched mode power supply circuits that convert an input DC voltage or AC voltage (such as line power) to a constant DC output voltage.
2. Description of Related Art
Switched mode power supply circuits use a combination of a switch and an inductor at the output stage. A Pulse Width Modulation (PWM) regulator turns the switch successively ON (conducting) and OFF (not conducting), for times T.sub.ON and T.sub.OFF respectively. A current through the inductor increases when the switch is ON, and decreases when the switch is OFF. The result is a constant DC voltage at the output stage.
Some power supply circuits need to sense the line voltage independently. This sensing is to assist in controlling the PWM regulator, and thus in controlling the times T.sub.ON and T.sub.OFF. These times are controlled such that the line current remains substantially in phase with the line voltage at each instant of the line voltage cycle. This will cause the current drawn from the power supply circuit to exhibit no harmonic distortion, and the input impedance of the circuit to be resistive only.
Although a resistive only input impedance is desirable, independent sensing of the line voltage is problematic. It is undesirable to use a resistive divider, because that would be connecting the high line voltage directly to the PWM regulator. This is even less desirable where the PWM regulator is to be implemented by an integrated circuit, as is preferred for control circuitry of that type. In some prior art embodiments, the line voltage is sensed by independent circuitry that has a frequency response fast enough to follow a 60 Hz sine wave variation. Such circuitry has been implemented in the prior art by a transformer. This implementation, however, needs to be implemented with large magnetic components to not produce error. Additionally, it can not be adapted to handle DC input power, because it relies on attributes of AC line power that are absent from a DC waveform.
U.S. Pat. No. 4,672,300 discloses a circuit where the switch is turned ON and OFF when the inductor current magnitude reaches lower and upper limits respectively. A PWM regulator is thus no longer necessary, as times T.sub.ON and T.sub.OFF are not controlled directly. In addition, this technique does achieve a resistive only input impedance, but only because it senses independently the rectified input line voltage, and uses it to set the upper limit of the inductor current. The lower limit of the inductor current is set advantageously at exactly zero. An inductor current with a lower limit at exactly zero is called "just discontinuous", and it results in additionally obviating the need for a snubber circuit, and in minimizing switching and inductor losses.